In the field of data processing, the use of point of sale (POS) terminals has required and also has enabled rapid and accurate transfer of information from one location to another in a business operation. In the case of department stores, the sales data must be transmitted or communicated from one or more terminals throughout the store to a central computer or processing unit.
A filing system is used to provide and maintain inventory and price data on the huge number of items that are sold in the store. The filing system and the control therefor provide means for retrieving and updating data and information regarding the constantly changing inventory and prices for the overall operation. The control for the POS system requires a means for controlling a line or circuit that carries data communication among individual terminals.
Representative documentation in the field of link control systems includes Japanese Patent Specification #659/88 which discloses a plurality of controllers and terminals connected together by use of an automatic line switcher. When a disorder occurs in a controller, the line switcher automatically switches the line to connect the terminal to another controller.
U.S. Pat. No. 4,366,535, issued to R. Cedolin et al. on Dec. 28, 1982, discloses a modular signal-processing system with a pair of identical microprocessors operating in parallel, one microprocessor being enabled to transmit messages while the other operates as a dummy. The two processors are interlinked by a connection enabling verification of correct operation under control of a common clock.
U.S. Pat. No. 4,468,750, issued to M. E. Chamoff et al. on Aug. 28, 1984, discloses a system of smart terminals and a dumb controller coupled by means of a communications link. Each of the terminals has a programmable digital processor, entry devices and display devices.
U.S. Pat. No. 4,502,120, issued to S. Ohnishi et al. on Feb. 26, 1985, discloses a system for data transmission between slave cash registers and a master register wherein a transmitting device divides the data into parts and the capacity of a receiving device is identical with the capacity of a memory device.
U.S. Pat. No. 4,626,844, issued to B. L. Mann et al. on Dec. 2, 1986, discloses an addressable electronic switch capable of series connection with a host computer and other peripheral devices. The switch circuit detects presence of an individual access code within the data stream and retransmits all data except its own access code.
In a conventional POS system, it is seen that when a communication control device, which is provided in a controller such as a master terminal or the like, is out of order and cannot control the communication of data, the line control is switched manually or is switched automatically by a line switcher. This method is used in a system which includes a backup master terminal that includes a communication control device that can be switched into the system and control the communication of data and thereby avoid down time of the system by reason of failure of a communication control device. The line switcher includes disorder detecting means for detecting disorder in the controller and switches the line so as to connect the terminal to another controller.
The above-mentioned manual line switching arrangement has a disadvantage in that the data communication among the terminals cannot be performed unless the operator finds the disorder and initiates action to effect or exercise switching the line. Additionally, the invention disclosed in the above-mentioned Japanese Specification has a problem in that the controllers and terminals are connected together through the line switcher, so that when the line switcher is out of order, the line cannot be successfully connected or controlled and data communication between the controllers and the terminals cannot be performed.
Further, a conventional POS system generally uses a four wire line or circuit. The use of the four wire line has a problem that in case the communication controlling operation is switched from the master terminal to the backup master terminal, it is necessary to switch the line connection between the receiver and the transmitter of the backup master terminal and a certain type line switcher is required in the operation.